That is from Advent of the Heart:
Seasonal Sermons and Prison Writings (Ignatius Press, 2006), by Alfred Delp, S.J., one of the very best books I've ever read on Advent. Fr. Delp was a German Jesuit priest who was imprisoned in Berlin in 1944, then tortured, imprisoned, and executed on February 2, 1945. The Advent meditations in this book were written while he was in prison, facing an almost certain execution. Free of any sort of false piety and sugary spirituality, they are challenging, bracing, and very timely. Fr. Delp's sermons and reflections, the Introduction notes, focus on four ways in which we are called in Advent to encounter God:
• We are called to integrity and authenticity
• We confess and proclaim our faith
* We respond to God with reverent awe
The following is from "The Mystery Made Present To Us," a pre-Christmas reflection preached in Munich, December 22, 1942, which is equally appropriate for Advent:
This brings us to the second point. Within all of the foregoing, a great mystery--the Mysterium--is hidden. Something happens between Heaven and earth that passes all understanding. This mystery is made present to us, continues in the world till the end of time, and is always in the process of happening--the abiding Mysterium.
These two points are followed by the third way in which we must consider the feast to be serious and important. Through the historical facts and through the workings of the mystery, the holy day simultaneously issues a challenge to each individual life, a message that demands a particular attitude and an interior decision from each person to whom it is proclaimed.
Read the entire piece. Also, see "Remembering Father Alfred Delp, S.J., Priest and Martyr", an interview with with Father Karl Adolf Kreuser, S.J., who knew Fr. Delp.
More excellent Advent reading can be found in Joseph Ratzinger's Seek That Which Is Above, which contains reflections on Advent, Christmas, and Easter, as well as other feasts. Advent, Christmas, and Epiphany in the Domestic Church, by Peter and Catherine Fournier, is filled with activities for children and families to help better understand and celebrate the Advent, Christmas and Epiphany seasons.